Engine Lubrication Method

ABSTRACT

A four-cycle engine has a lightweight aluminum alloy engine block having a cylindrical bore and an enclosed oil reservoir formed therein. A crankshaft is rotatably mounted in the engine block for rotation about a crankshaft axis. A piston reciprocates within the bore and is connected to the crankshaft by a connecting rod. An oil pump driven by the cam gear, which mates with crank gear that is driven by crank shaft, inhales the oil from the oil reservoir and the valve chamber to splash lubricate into the cylinder bore. The engine is provided with a cylinder head assembly defining a compact combustion chamber having a pair of overhead intake and exhaust ports and cooperating intake and exhaust valves.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an engine, and more particularly, an engineconstruction and lubrication method for a small four-cycle internalcombustion engine which is particularly suitable for the use withportable or transportable power tools.

2. Description of the Related Art

U.S. Pat. No. 7,287,508 to Kurihara and U.S. Pat. No. 7,624,714 toKurihara et al., which are incorporated herein by reference, disclose alight weight and compact prior art small four-cycle engine construction.

Portable power tools such as line trimmers, blower/vacuums, chain sawsare mostly powered by two-cycle internal combustion engines or electricmotors. Some transportable power tools such as tiller/cultivators,generators are currently powered by two-cycle or four-cycle internalcombustion engines. With the growing concern regarding air pollution,there is increasing pressure to reduce the emissions of both portableand transportable power equipment. Electric motors unfortunately havelimited applications due to power availability for corded products, andbattery life and power availability for cordless devices. In instanceswhere weight is not an overriding factor such as lawn mowers, emissionscan be dramatically reduced by utilizing heavier four-cycle engines.When it comes to power tools such as line trimmers, chain saws andblower/vacuums, four-cycle engines pose a very difficult problem.Four-cycle engines tend to be too heavy for a given horsepower outputand lubrication becomes a very serious problem since portable ortransportable power tools must be able to run in a very wide range oforientations except generators or tiller/cultivators. For sometiller/cultivators powered by four-cycle engines with vertical powershafts, lubrication also becomes a serious problem since it is difficultto use the same lubrication system as engines with horizontal powershafts.

Therefore, it is an object of the present invention to provide a smallfour-cycle internal combustion engine having low emissions that issufficiently light weight to be carried and/or transported by anoperator, which is especially suitable for a hand-held or transportablepower tool.

It is a further object of the present invention to provide a smallfour-cycle internal combustion engine having an internal lubricationsystem enabling the engine to be run at a wide variety of orientationstypically encountered during normal operation, which is especiallysuitable for a portable or transportable power tool.

It is a further object of the present invention to provide a smalllightweight four-cycle engine having an engine block, an overhead valvetrain and a lubrication system to splash oil mist to lubricate thecrankshaft chamber throughout the normal range of operating positions,which is especially suitable for a portable or transportable power tool.

It is a further object of the invention to provide a lubricant returnsystem to return lubrication oil into oil reservoir after lubricatingparts in the crankshaft chamber and the overhead valve chamber.

In the prior art previously referred, circular and scroll-type wallsplay important roles in the inventions. However, the construction tomake these walls are not so cost effective and not so compact. Thus, itis a further object of the present invention to provide a more costeffective and more compact construction.

These and other objects, features, and advantages of the presentinvention will become apparent upon further review of the remainder ofthe specification and the accompanying drawings.

SUMMARY OF THE INVENTION

In order to achieve the above objects, a four-cycle, internal combustionengine is provided which is suitable for use with portable ortransportable power tools. The four-cycle engine is provided with anengine block having at least one cylindrical bore oriented in a normallyupright orientation having an enclosed combustion chamber.

A lower case is attached to the said cylinder block with a horizontalmating plane. The cylinder block and said lower case form a crankshaftchamber. A crankshaft is pivotably mounted within the crankshaftchamber. An enclosed oil reservoir is located below the crankshaftchamber separated from the crankshaft chamber by a substantiallycircular wall.

A pump is connected drivably to cam gear-cam assembly, and said pumpinhales lubrication oil from the oil reservoir to splash oil into thecylinder.

When lubrication oil is properly filled, the engine is able to rotate orto be stored without oil flowing into combustion chamber at anyinclination posture.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional side elevation view of the engine takenalong the rotating axis of the crankshaft and axis of cylinder boretaken along line I-I in FIG. 2.

FIG. 2 is a cross-sectional side elevation view of the engine takenalong line II-II in FIG. 1;

FIG. 3 is an enlarged schematic illustration of the camshaft and thefollower mechanism;

FIG. 4 is a cross-sectional side elevation view of the engine of FIG. 2when it is oriented to be upside down.

FIG. 5 is a cross-sectional side elevation view of the engine of FIG. 1when it is oriented to be upside down.

FIG. 6 is a section view of the oil pump cover that shows the detailconstruction of inlet and outlet cavities of the pump taken along lineVI-VI in FIG. 11.

FIG. 7 is a section view of the cylinder block and the lower case takenalong line III-III in FIG. 1.

FIG. 8 is an elevation view of the cylinder block viewing from A in FIG.7.

FIG. 9 is a cross-sectional side elevation view of the engine of FIG. 1when it is oriented with the power take off end down.

FIG. 10 is a cross-sectional side elevation view of the engine of FIG. 1when it is oriented with the power take off end up.

FIG. 11 is an enlarged view of the pump shown in FIG. 1.

FIG. 12 is a cross-sectional side elevation view of the engine accordingto another embodiment of the present invention.

FIG. 13 is a cross-sectional side elevation view of the engine of FIG.12 when it is oriented to be upside down.

DESCRIPTION OF THE EMBODIMENTS

FIGS. 1 and FIG. 2 illustrate a cross-sectional side elevation view of afour-cycle engine. The four-cycle engine is made up of a lightweightaluminum housing including a cylinder block 1 having a cylindrical bore2 formed therein.

A lower case 3 and said cylinder block 1 mate with each other at thehorizontal interface and form a crankshaft chamber 5.

A crankshaft 4 is pivotably mounted within the crankshaft chamber 5 in aconventional manner. The axis of said crankshaft 4 is contained at thehorizontal interface to mate cylinder block 1 and lower case 3.

A piston 6 slides within the cylindrical bore 2 and is connected to thecrankshaft 4 by a connecting rod 7. A cylinder head 8 is integrated tothe engine block 1 to define an enclosed combustion chamber 9.

The cylinder head 8 is provided with an intake port 10 coupled to aninsulator 11 and a carburetor 100 and selectively connected to thecombustion chamber 9 by an intake valve 12. A filter element of aircleaner 101 eliminates dust from the intake air into the engine. Thecylinder head 8 is also provided with an exhaust port 13 connected to amuffler 14 and selectively connected to the combustion chamber 9 by anexhaust valve 15.

As illustrated in FIGS. 1 and 2, the cylinder axis 16 of four-cycleengine is generally upright when in normal use.

The lower case 3 is connected to a bottom cover 17 that provides anenclosed oil reservoir 18.

The crankshaft 4 is provided with an axial shaft member 19 having anoutput end 20 adapted to be coupled to a flywheel 21 which has animplement input member 22. An input end 23 of axial shaft member 19 iscoupled to a counterweight web 24. A crankpin 25 is affixed tocounterweight webs 24, 26 and is parallel to and radially offset fromthe axial shaft 19. The crankpin 25 pivotally cooperates with a rollerbearing 27 (FIG. 2) mounted in connecting rod 7.

The axial shafts 19 and 28 of crankshaft 4 are pivotably attached tobetween the cylinder block 1 and the lower case 3 by a pair of bearings29 and 30. At the side of bearing 30, a crank gear 31 is mounted on thecrankshaft 4 in a cam chamber 53.

A camshaft drive and valve lifter mechanism is best illustrated in FIGS.1 and 3. The crank gear 31 mounted on the crankshaft 4 in turn drives acam gear 32 with twice the number of teeth as the crank gear 31,resulting in the camshaft 33 rotating in one-half engine speed. The camgear 32 is affixed to a camshaft 33 which is journaled to the cylinderblock 1 and includes a rotary cam lobe 34. In the embodimentillustrated, a single cam lobe is utilized for driving both the intakeand exhaust valves. Followers 35 and 36 are pivotably connected to thecylinder block 1 by a pivot pin 37.

Push rods 38 and 39 extend between camshaft followers 35 and 36 androcker arms 40 and 41 located within the cylinder head 8. The cam 34,push rods 38, 39 and rocker arms 40, 41 are part of a valve trainassembly. Affixed to the cylinder head 8 is a valve cover 42 whichdefines therebetween an enclosed valve chamber 43.

A wall 44 surrounds the intake and exhaust push rods 38 and 39 in aconventional manner in order to prevent the entry of dirt into theengine.

The cam chamber 53 and the valve chamber 43 are in communication witheach other.

In order to lubricate the engine, a pump 45 such as a trochoid pump isplaced at the side of cam gear 32. As illustrated in FIG. 1, FIG. 6 andFIG. 11, the pump 45 is formed of a stationary pump cover 120 comprisingan inner portion 121 mounted on the camshaft 33 and a stationary pumpbody 122 comprising an annular outer portion 123 coaxially disposedaround the inner portion 121 with a certain distance from the innerportion 121, an inner rotor 46 formed on the outer surface of thecamshaft 33 and an outer rotor 47 formed on the inner surface of theouter portion of the pump body. In other embodiments of the presentapplication, a gear pump or plunger pump may be used.

The inner rotor 46 formed on the outer surface of the camshaft 33 isdriven by the cam gear 32, and the outer rotor 47 formed on the innersurface of the outer portion 123 is rotated following the rotation ofthe inner rotor 46. Lubrication oil is inhaled from the passage 48. Oneend of the passage 48 leads to an oil entrance of the pump. The otherend of passage 48 is connected to one end of a flexible tube 49. Theother end of flexible tube 49 is connected to a filter with weight 50.By means of the weight 50, an entrance of the flexible tube 49 is dippedin the oil in the oil reservoir 18 at any orientation of the engine.

The oil pushed out by the pump 45 is lead in parallel to a hole 52formed in the cylinder wall and a hole 125 formed in the outer wall ofthe cam shaft 33 through an inner through hole 51 of the cam shaft asillustrated in FIG. 1. Accordingly, the engine parts inside thecrankshaft chamber 5 and the cam chamber 53 are mist lubricated by theoil splashed by means of the rotation of and/or the centrifugal forcegenerated by the rotating parts such as web 24, 26.

As illustrated in FIGS. 1 and 2, a circular arc wall 55 surrounding thecounterweight web 24,26 of the crankshaft 4 separates crankshaft chamber5 from the oil reservoir 18. The arc wall 55 is substantially co-axialwith the axis of the counterweight web 24 or 26 and is located with acertain distance from the webs 24 and 26.

At the bottom of circular arc wall 55, a drain hole 56 is provided. Asillustrated in FIG. 1, in the section of circular arc wall 55 by planeswhich are parallel to a plane including crankshaft axis and cylinderaxis 16, circular arc wall 55 is 20-45 degrees inclined to the matingsurface of cylinder block 1 and lower case 3 to make a funnel ascylinder axis 14 is a mirror axis. Slit 57 and 58 are provided at thebottom of circular arc wall 55 to communicate with the crankshaftchamber 5 and the oil reservoir 18.

At the side of the cylinder block 1, a drilled oil passage 104 isprovided. One end of the passage 104 leads to the oil entrance of thepump together with the passage 48. The other end of passage 104 leads toupper portion in the valve chamber 43 as illustrated in FIG. 1.

A small hole 109 is opened from the valve chamber to the passage 104near the bottom surface of the valve chamber.

In other embodiments of the present application, a flexible tube may beused to provide passage 104. An oil inlet 107 is provided at said otherend of the passage 104.

As best illustrated in FIG. 6 and FIG. 11, the pump 45 has a first inletcavity 200 which inhales oil from the oil reservoir 18 through thepassage 48 and a second inlet cavity 201 which inhales oil from thevalve chamber 43 through the passage 104.

Between the first inlet cavity 200 and the second inlet cavity 201, awall 202 is provided to separate the cavities 200 and 201. An outletcavity 203 provides a passage for oil to the cylinder bore through theinner hole 51. A relief oil passage is formed in the outer portion 123of the pump body 122. One end of the relief oil passage 204 opens to theoutlet cavity 203. The other end of the relief oil passage 204 opens tothe oil reservoir 18 through a drilled hole (not shown).

In the valve chamber 43, a breather pipe 61 is opened through the valvecover 42. One end of the breather pipe 61 is connected to an air cleanercase 62 through a breather tube 63. At said one end of breather pipe 61,a check valve 60 is installed. The valve chamber 43 opens when thepressure in the valve chamber 43 is higher than the pressure in thebreather tube 63, and the valve chamber 43 closes when the pressure inthe valve chamber is lower than the pressure in the breather tube 63. Inthe air cleaner case 62, an oil separating deflector 102 is provided.The breathing oil mist provided through the breather tube 63 isseparated into oil-lean gas and oil-rich gas by the deflector 102.

A return tube 64 interconnects the air cleaner case 62 and a return hole65 formed in the cylinder wall. The return hole 65 opens and closes witha reciprocating motion of the piston 6 and the oil-rich mist returnsinto the crankshaft chamber 5 only when the pressure in the crankcase isnegative. The oil-lean mist is inhaled to the carburetor 100 through thefilter element 101.

As illustrated in FIG. 2, around the outside wall of cylinder bore inthe crankshaft chamber 5, oil recess 75 and 76 are provided.

FIG. 7 is a section view of the cylinder block 1 and lower case 3 at theportion where the bearing 29 or 30 is supported. FIG. 8 is an elevationview of the cylinder block 1 seen from A illustrated in FIG. 7.

As illustrated in FIG. 1, another oil recess 77 is provided at theoutside of the bearing 29. The oil recess 77 and the oil recesses 75, 76of the crankshaft chamber 5 are communicated by slits 78 and 79 asillustrated in FIGS. 7 and 8.

By the same way, in the boss to support bearing 30, slits 80 and 81 areprovided to communicate between the crankshaft chamber 5 and cam chamber53.

By the reciprocating motion of the piston 6, pressure in the crankshaftchamber 5 changes up and down and oil mist in the crankshaft chamber 5is pushed out to the cam chamber 53 and lubricate the valve actuatingparts in the cam chamber 53.

As illustrated in FIG. 1, at the bottom of the cam chamber 53, a pipe 54is provided. One end of the pipe opens to the cam chamber 53 and theother end of the pipe 54 opens to oil reservoir 18 with a small distancefrom the wall of the bottom cover 17.

The axes of intake valve 12 and exhaust valve 15 are inclined to eachother. Therefore, a corner 82 of a deck 84 of the cylinder head 8 nearthe inlet valve spring 83 in the valve chamber 43 is lower than the topof the deck 84. So, in the normal operation posture of the engine inwhich the cylinder head 8 is upright, lubricating oil in the valvechamber 43 flows easily to the corner 82 of the deck 84 afterlubricating the parts in the valve chamber.

A passage 85 is provided between the corner 82 of the deck 84 and theoil reservoir 18. At one end of the passage 85, a pipe 86 is provided inthe oil reservoir 18 with a small distance from the wall of the bottomcover 17. Since intake and exhaust valves 12, 15 are inclined to eachother, cooling performance of the engine in accordance with the presentinvention is better than that of the prior art because a cooling airpassage 124 between valves is wide.

Other parts not specifically referenced in the foregoing relate toconventional four-cycle engines. As illustrated in FIG. 1, a spark plug66 is installed in a spark plug hole formed in the cylinder head 8. Acoil 67 is an ignition coil. A re-coil starter 68 having a re-windingrope 69 is provided at a side of crankshaft 4. At the lower corner ofthe lower case 3, a cooling air entrance 70 is provided which inhalescooling air for the engine generated by rotation of blade 71 on theflywheel 21.

A fuel tank 72 is provided below the oil reservoir 18, adequately spacedapart therefrom. As illustrated in FIG. 2, in the fuel tank 72, a fuelfilter 73 and a fuel pipe 74 are provided to inhale fuel into thecarburetor 100 therethrough.

In order to achieve high power output and relatively low exhaustemissions, the four-cycle engine in accordance with the presentinvention is provided with a very compact combustion chamber 9. When theengine is started by pulling the winding rope 69 illustrated in FIG. 1,lubricating oil is immediately inhaled to the oil pump 45 by rotation ofthe rotors 46, 47 through flexible tube 49. Lubricating oil is splashedinto the cylinder bore through the holes 51 and 52 and into the camchamber 53 through the slits 80 and 81. By means of the weight 50supported by and connected to the flexible tube 49, oil is inhaled atany position of the engine. The oil mist in the cam chamber 53, in whichthe valve actuating parts are installed, lubricates the valve train andthen flows into the air cleaner box through the passages 61 and 63 asshown in FIG. 2. When the pressure in the cylinder bore is negative, aport 65 formed in the wall of cylinder bore opens and the mist returnsfrom the air cleaner case 62 into the cylinder bore through passage 64.

The excess oil after lubricating parts in the valve chamber 43 returnsinto oil reservoir 18 through the passage 85 shown in FIG. 2.

As illustrated in FIGS. 1 and 2, the circular arc wall 55 surrounds thecounterweight webs 24, 26 at a slight distance from the web. Thecrankshaft webs 24 and 26 splash the oil to mist lubricate the internalengine parts.

After lubricating the engine parts, as the webs 24, 26 rotate, the oilreturns into the oil reservoir 18 through the drain hole 56.

The excess oil in the cam chamber 53 returns into the oil reservoir 18through the pipe 54.

Portable or transportable power tools are operated with variousorientations. For instance, a typical brush cutter, which installs anengine at an upper end of the boom and a cutter at the lower oppositeend of the boom, is usually operated with 25-40 degrees inclination ofthe boom relative to the ground.

In the present invention, the circular arc wall 55 is funnel shaped witha 20-45 degree conical angle. Therefore, lubricating oil is not agitatedexcessively by rotation of web 24 and drops into the oil reservoir 18when a brush cutter is operated in normal operation posture.

As illustrated in FIGS. 4 and 5, even when the engine is positionedupside down, lubrication oil in the oil reservoir 18 is kept in ithelped by the circular arc wall 55 and oil is prevented from flowinginto the cylinder head part. Further, oil in the crankshaft chamber 5 iskept in the oil recesses 75, 76 (FIGS. 4) and 77 (FIG. 5) and oil isprevented from flowing into combustion chamber 9.

As illustrated in FIGS. 4 and 5, the pipe 54 (FIG. 5) prevents oil fromflowing into cam chamber 53 and the pipe 86 (FIG. 4) prevents oil fromflowing into valve chamber 43 when the engine is oriented upside down.

As illustrated in FIG. 1, when the engine is in a normal orientation,the lubricating oil is inhaled from the oil reservoir 18 through thesmall hole 109 into the valve chamber 43. Further, as illustrated inFIG. 5, when the engine is positioned upside down, the oil, afterlubricating various parts in the valve chamber 43, is inhaled by pump 45from the oil inlet 107 and sent to the oil pump 45 through the passage104. Accordingly, excess oil does not remain in the valve chamber 43.

It is necessary to be sure that portable or transportable power toolsare safely stored. For instance, even if a brush cutter is stored with aposture with the vertical boom side up and the engine side down,lubrication oil should not flow into the combustion chamber 9. On thecontrary, even if the same brush cutter is stored with a posture withthe vertical boom side down and the engine side up, lubrication oilshould not flow into the combustion chamber 9 also.

FIG. 9 illustrates storage of the engine when the output side of theengine is orientated to be down. Oil A is kept in the oil reservoir 18and does not flow into crankshaft chamber 5. Oil B is kept in the recess77 in the cylinder block 1 and does not flow into the crankshaft chamber5.

FIG. 10 illustrates storage of the engine when the output side of engineis oriented to be up. Oil is kept in the cam chamber 53 and does notflow into crankshaft chamber 5.

5

The function of the relief passage 204 the pump 45 is as follows:Assuming that the sectional area of oil passage of the hole to splashoil into cylinder bore is S₁, the sectional area of relief passage iss₂, and the discharge volume by pump is Q. Then, the discharge volume into the cylinder bore is Q*S₁/(S₁+S₂). Return volume of oil to the oilreservoir is Q*S₂/(S₁+S₂). So, by adequate design, in accordance withthe present invention, some part of the oil discharged by the pump 45always returns into the oil reservoir 18. In the prior arts, on theother hand, because there is provided no relief passage, in someoperation condition, all of the oil in the oil reservoir 18 is suckedand sent into the cylinder bore, and no oil returns into the oilreservoir 18. This means that there may be a rare instance where thecrankshaft chamber 5 is filled by a lot of oil. The present inventionsolves this problem.

FIGS. 12 and 13 show another embodiment of the present invention,wherein a passage 85 is provided between the corner 82 of the deck 84and the oil recess 76 in the cylinder block. At one end of the passage85, a pipe is provided in the oil recess with a similar height with theupper wall 127 of the crankshaft chamber 5. As illustrated in FIG. 13,the pipe 125 prevents oil from flowing into the valve chamber 43 whenthe engine is oriented to be upside down.

It is believed that small light weight four cycle engines made inaccordance with the present invention will be particularly suitable forthe use with hand-held or transportable power tools having low emissionsand will be sufficiently light to be carried and/or transported by anoperator. In the prior art, effective lubricating methods for hand-heldor transportable power tools have been presented. However, they requirecomplicated arc and scroll shaped wall to control flow of lubricatingoil in the engines and to prevent oil from flowing into cylinder headwhen engine is positioned upside down or vertical.

In the present invention, however, more simple and economic constructionis provided to solve the problem in the prior arts.

Further, construction such as the pump in the prior art is basicallyfollowed, so that specific feature and advantage of the prior art aremaintained in the present invention.

1. A single-cylinder, four-stroke cycle, spark ignition internal combustion engine for mounting on a power tool comprising: a cylinder block having a cylinder, a cylinder head, a piston mounted for reciprocation in said cylinder, said cylinder head defining an air-fuel combustion chamber; a lower case mating with said cylinder block by a plane perpendicular to the cylinder bore axis; a crankshaft chamber formed by said cylinder block and said lower case; an air-fuel mixture intake port and an exhaust gas port in said cylinder head; a valve cover on said cylinder head defining a valve chamber; an intake valve and an exhaust valve mounted in said intake and exhaust port, respectively, for reciprocation between port-open and port-closed positions; a valve-actuating valve train in a valve chamber, said valve train including at least one rocker arm and at least one valve train push rod assembly extending therefrom within said valve chamber and engaging said rocker arm; a crankshaft rotatably mounted in said crankshaft chamber, wherein said crankshaft includes a crank portion and at least one counterweight web and has an axis which is on the mating plane of said cylinder block and said lower case; a connecting rod having articulated connections at one end thereof to said piston and at an opposite end thereof to said crank portion, thereby forming a piston-connecting rod crankshaft assembly; a cam chamber having at least one earn being drivably connected to said crankshaft, said at least one cam having a cam gear and being driven at one-half crankshaft speed, the opposite end of said push rod assembly being drivably connected to said cam whereby said push rod assembly is actuated with a reciprocating motion upon rotation of said at least one cam, said cam chamber having communication means to the crankshaft chamber; a lubrication oil reservoir formed below the lower case; and an oil pump connected drivably to said cam gear-cam assembly, wherein said pump inhales lubrication oil from said oil reservoir and valve chamber and splashes the oil in parallel into the cylinder and into the cam chamber to lubricate the engine parts inside the cylinder and the cam chamber and the valve chamber.
 2. The engine set forth in claim 1 further comprising an air cleaner box connected to said valve chamber via a first passage through which breathing oil mist gas flows, a second passage connecting the air cleaner box to the crankshaft chamber or cylinder block, and a valve being provided at the entrance of said passage into the crankshaft chamber, wherein the opening of the valve is controlled by reciprocating motion of said piston, and wherein said valve opens when pressure in the crankshaft chamber is negative and closes when the pressure in the crankshaft chamber is positive, thereby the oil mist flow control valve structure establishing a lubrication oil mist flow circuit from said valve chamber to said crankshaft chamber or said cylinder block through said air cleaner box.
 3. The engine set forth in claim 1, wherein said oil pump is integrally attached with the cam or the cam gear.
 4. The engine set forth in claim 1, wherein said oil pump is a trochoid pump.
 5. The engine set forth in claim 1, wherein said oil pump has two separated inlet cavities.
 6. The engine set forth in claim 1, wherein said oil pump has at least one relief passage which communicates between outlet cavity of the pump and the oil reservoir.
 7. The engine set forth in claim 1, wherein said lower case has a circular arc wall to separate said oil reservoir from said crankshaft chamber, said circular arc wall at least partially surrounding said web at a slight distance therefrom, and said circular arc wall having a drain hole and slits to communicate between said crankshaft chamber and oil reservoir.
 8. The engine set forth in claim 1, wherein said circular arc wall is shaped to be a 20 to 45 degree to said drain hole.
 9. A hand-held, transportable, or stationary power tool driven by the engine set forth in claim 1, wherein said power tool is driven by a horizontal or vertical or inclined power shaft.
 10. The engine set forth in claim 1, wherein said cylinder block has at least one oil recess to store oil when the engine is oriented to incline and to prevent oil in the crankshaft chamber from flowing into combustion chamber.
 11. The engine set forth in claim 1, wherein said lower case has at least one oil recess to store oil when the engine is oriented to incline and to prevent oil in the oil reservoir from flowing into said combustion chamber.
 12. The engine set forth in claim 1, wherein said lower case has at least one long pipe to prevent oil in the oil reservoir from flowing into said cam chamber or said valve chamber.
 13. The engine set forth in claim 1, wherein said intake and exhaust valves are inclined with respect to each other.
 14. The engine set forth in claim 1, wherein said cylinder block has at least one oil passage located near the intake valve to communicate between said valve chamber and said oil reservoir.
 15. The engine set forth in claim 1, wherein said cylinder block has at least one oil passage located near the intake valve to communicate between said valve chamber and said oil recess in the cylinder block.
 16. The engine set forth in claim 15, wherein said cylinder block has at least one long pipe to prevent oil in the oil recess from flowing into said valve chamber. 